The halo usually appears around the Sun and Moon, sometimes around other powerful light sources such as street lights. There are many types of halos and they are mainly caused by ice crystals in cirrus clouds at an altitude of 5-10 km in the upper troposphere.
The type of halo depends on the shape and location of the crystals. Light reflected and refracted by ice crystals often decomposes into a spectrum, which makes the halo look like a rainbow. Parghelia and zenith arch are the brightest and most full-color ones, the tangents of the small and large halos are less bright.
In the dim moon halo of colors, the eye is not visible, which is associated with the peculiarities of twilight vision.
Sometimes in frosty weather, a halo is formed by crystals very close to the earth's surface. In this case, the crystals resemble shining gems (called diamond dust), and the lower part of the halo can be seen against the background of the surrounding landscape if the Sun is low enough above the horizon.
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